Passive frequency standards comprise in a known manner storage means, subjected to a uniform static magnetic field, for storing atomic or molecular elements in gaseous form, means for producing a population inversion by putting these elements into a predetermined energy level, an interrogation means for stimulating at a predetermined frequency a transition of the energy level of such elements and slave means for controlling the frequency of the interrogation means as a function of a signal depending on the radiation emitted by the stimulated transition.
The general principle of operation of frequency standards and in particular passive frequency standards is particularly described in the article "Les Horloges Atomiques" by C. Steiner, which appeared in the February/March 1967 issue of the review "La Suisse Horlogere" published by the Chambre Suisse de l'Horlogere.
Embodiments of a passive atomic frequency standard using a gas cell and a passive maser are described respectively in Swiss patent document CH-A-40 370 and U.S. Pat. No. 4,316,153.
By way of example of a known passive frequency standard, there will now be described an atomic frequency standard employing a gas cell having reference to FIG. 1.
This frequency standard essentially comprises an optical pumping arrangement 10, a quartz oscillator 12 and an electronic circuit 14 which compares the resonance frequency of the atomic system to that of the oscillator for the slaving of the latter.
In the arrangement 10 there is produced by optical pumping a population inversion between the hyperfine levels of the ground state of the atoms which are generally alkali metals such as potassium, sodium or rubidium. In the case of a frequency standard using rubidium, a standard optical pumping set up is the following.
A cell 16 contains the isotope Rb 87 the spectrum of which comprises the two hyperfine components A and B. The cell is illuminated by a rubidium 87 lamp 18 through a filter 20 which contains the isotope Rb 85 the absorption spectrum of which includes the hyperfine components a and b. The components A and a exist practically in coincidence, while the components B and b are completely separated. The component A of the emission spectrum of lamp 18 is thus eliminated by the filter 20 in a manner such that the light which attains cell 16 is for the most part constituted by the band B. Only the atoms of Rb 87 of cell 16 situated in the lower hyperfine level (F=1) absorb light and are transported into higher states. They return by spontaneous emission either to the upper hyperfine level (F=2) or to the lower hyperfine level. Since these atoms are immediately excited by the arrival of the light, the lower level (F=1) is depopulated to the benefit of the upper level (F=2). There is thus brought about the population inversion between these two levels and thereby cell 16 becomes practically transparent to radiation from lamp 18.
Cell 16 is arranged in a microwave cavity 22 which is excited to a frequency close to 6835 MHz, corresponding to the separation energy of the hyperfine levels F=1, m.sub.F =0 and F=2, m.sub.F =0 which brings about the hyperfine transition accompanied by an emission of electromagnetic radiation between these two levels. As soon as the atoms which participate in the stimulated emission arrive at the lower hyperfine level (F=1), they are optically pumped and transported into the excited states.
The greater the number of stimulated transitions, the greater will be the quantity of light absorbed in cell 16, the smaller will be the quantity of light arriving at the photoelectric cell 24 and the smaller the current will be in photoelectric cell 24. Therefore, the photoelectric current goes through a minimum when the frequency of the excitation signal of the cavity is exactly equal to the transition frequency.
The quartz oscillator 12 produces a signal at 5 MHz. This signal is modulated in a phase modulator 26 to a relatively low frequency (typically on the order of 100 Hz) produced by a low frequency generator 28.
The modulated signal is applied to a multiplier synthesizer 30 to obtain a signal having the stimulated emission frequency of 6835 MHz. It is this signal which is employed in order to excite the microwave cavity 22.
The signal furnished by the photoelectric cell 24 is received by an amplifier 32, then applied to a phase comparator 34 which also receives a reference signal from the generator 28 in a manner to bring about a synchronous detection enabling determination of whether the carrier frequency of the signal applied to cavity 22 is well centered on the hyperfine transition frequency (6835 MHz). Any shifting is indicated by an error signal at the output of the phase comparator 34. This signal is sent to an integrator 36, then employed in order to control a variable capacitor 38 coupled to the oscillator 12 and which modifies the frequency of the latter so as to maintain the multiplied frequency of the quartz centered onto the frequency of the hyperfine transition of the rubidium.
The classic frequency standard shown on FIG. 1 includes a cylindrical resonant cavity 22. The latter constitutes a limitation to the miniaturization of the frequency standard.
Effectively, the resonant cavity must have dimensions largely superior to those of the cell in order that the latter is subjected to an essentially axial field. Furthermore, the dimensions of the resonant cavity may not be freely chosen, but depend, to the contrary, on the desired resonance frequency.
FIG. 1 illustrates the general structure of a passive frequency standard of the gas cell type. Passive masers exhibit the same type of limitation as far as miniaturization is concerned. Effectively, the optical pumping arrangement is replaced in passive masers by an arrangement comprising a storing bulb pierced with an opening and placed into a resonant cavity, a source of gas such as atomic hydrogen emitting a beam of particles towards the opening of the storage flask and a state selector means placed within the trajectory of the beam and which deviates the particles which are not in a predetermined energy state. One thus obtains a population inversion in the storing bulb. The arrangement also comprises a coupling loop for detecting radiation emitted by the stimulated emission.
Passive masers and gas cells thus both include a resonant cavity which limits the miniaturization possibilities for these frequency standards.
The purpose of the invention is to render passive frequency standards more compact. This purpose is attained by a new structure of the resonant means.